Tannin, or tannic acid, is a complex natural organic compound found in many plants, mainly in roots, bark, leaves, and some fruits. It has a characteristic astringent taste and can cause a distinctive sensation of dryness in the gums. For example, it is responsible for the astringency, color, and some of the flavors in tea. Tannins are particularly abundant in pomegranates, persimmons, apples, blueberries, blackberries, cherries, cranberries, and so on. Tannins, which are soluble in water, can form dark blue or dark green solutions with iron salts, and due to this property are used to make iron gall ink. Tannins are an important ingredient in the process of tanning leather, which is necessary to increase its plasticity, strength, and durability.